The present invention relates to hydrostatic drive systems and particularly to a control for protecting hydrostatic drive units of vehicle transmissions from cold weather startup damage.
Hydrostatic drive systems, such as hydrostatic transmissions and hydrostatic drive units in hydromechanical transmissions, are designed to operate safely with hydraulic fluids having a viscosity within a predetermined range. If the temperature of the hydraulic fluid is extremely low, its viscosity increases to the point where it does not properly flow through hydraulic circuits. A hydraulic pump, driven by the vehicle engine, in pumping cold, highly viscous hydraulic fluid, can over-pressurize the hydraulic circuit. Moreover, the hydraulic pump and hydraulic motor in the hydraulic circuit are not adequately lubricated when the hydraulic fluid is too cold. Due to differentials in expansion coefficients, clearances between component parts subjected to extremely low temperature hydraulic fluid can go out of tolerance. All of these factors have the potential of causing transmission malfunctioning at the least or, at the worst, permanent damage to hydraulic components if the vehicle, after standing long periods in extremely cold weather, is driven without an adequate hydraulic fluid warm-up period.
One approach to avoiding potentially damaging cold startups has been to utilize auxiliary heaters to warm the hydraulic fluid to an acceptable viscosity or to drive the transmission at a low idle speed to produce circulation and thus warming of the hydraulic fluid. Vehicle propulsion is inhibited until a thermostatic element senses that the hydraulic fluid temperature has risen to a safe operating level. An auxiliary heater requires an auxiliary energy source which is likely not be available in the field. Moreover, thermostatic elements are not particularly reliable when subjected to the hostile environment within a transmission housing.